Automatic electrode control

ABSTRACT

An electrode-positioning system for electric furnaces including a first circuit which develops a signal proportional to the magnitude of voltage at the electrode, and a second circuit which develops a signal proportional to the magnitude of current flow through the electrode. Outputs of each of the two circuits are impressed upon the power supply circuit of a reversible winch motor through saturable core reactors. The relative magnitudes of the control signals govern the direction of current flow through the motor supply circuit, and the winch motor, in turn, raises or lowers the electrode through a direct mechanical coupling.

United States Patent 1 Bennett AUTOMATIC ELECTRODE CONTROL [76]Inventor: George A. Bennett, 620 18th St.,

Beaver Falls, Pa. 15010 22 Filed: Aug. 17,1972

[21 Appl. No.: 281,316

[52] U.S..Cl. 13/13 [51] Int. Cl H05b 7/00, F27d/1l/10 [58] Field ofSearch 13/13 [56] References Cited UNITED STATES PATENTS 3,277,22910/1966 Oppenheim 13/13 Oct. 23, 1973 Primary Examiner-Roy N. Envall,Jr.

Attorney-Jacob Trachtman ABSTRACT An electrode-positioning system forelectric furnaces including a first circuit which develops a signalproportional to the magnitude of voltage at the electrode,

electrode through a direct mechanical coupling.

3 Claims, 3 Drawing Figures AUTOMATIC ELECTRODE CONTROL BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates toelectrode-positioning controls for electric furnaces and the like.

- 2. Description of the Prior Art It is conventional to use electricfurnaces to develop high temperatures for melting, processing or holdingthe temperature of metals. In such furnaces, electric current is usedeither to form an arc between an electrode and the metal charge in thefurnace to reduce the charge to a molten state, or to provide heat via apath between a submerged electrode and molten metal in the furnacehaving a high resistance to the current flow. The former are termeddirect arc furnaces and the latter are termed submerged arc furnaces. Inthis connection, alternating as well as direct current may be used, butin view of its availability and for other reasons, the trend has beentoward the use of conventional alternating current,three-phaseelectrical power.

It is highly necessary and desirable to control the positioning of theelectrodes in these furnaces for several reasons. In direct arcfurnaces, it is necessary to compensate for attrition of the electrodetip by moving the electrode downwardly toward the charge. Also, whenmelting low density charges such as scrap materials, it is essentialthat the electrode follow the charge as it becomes molten and gravitatesdownwardly to the bottom of the furnaces away from the electrode, inorder to maintain a fixed, optimum arc length for efficiently meltingthe material. In submerged arc applications, control over the depth ofpenetration of the electrode into the charge is necessary in order tofix the current at which the charge is melted. In both types offurnaces, it is economically desirable to control the position of theelectrodes with respect to the charge in order to prevent large laggingpower factors and attendant high KVA levels'resulting from high currentlflows, as the occurrence of this results in high electrical power costsin relation to usable power consumed.

Some systems in the past have used a motorgenerated or amplidyne unit toprovide power for the electrode winch motor. Electrode voltage andcurrent sensing circuits are used to impress signals of varying polarityto the field windings of the generator, thus controlling the directionof current flow in the winch motor supply circuit which determines thedirection of rotation of the winch motor. Of course, the motorgeneratoror amplidyne units add a significant amount to the initial cost of thecontrol system, and further, increase the number of elements requiringmaintenance.

Hydraulic systems have also been proposed. Also, other types of systemshave been proposed, all of which include an intermediaryelectro-mechanical or mechanical element which controls or operates inconjunction with the electrode winch motor. These systems haveincorporated such items as relays, motordriven switches with rotatablewipers, differentials, and electrically controlled clutches all of whichadd to the initial and operating costs of these systems.

Accordingly, it is an object of this invention to develop an electrodecontrol system that will meet the above factors and enable a wider ormore extended use of proportional control as distinguished from a manualcontrol.

Another object of this invention is to devise a control system forelectrode operative positioning which will eliminate the need forrotary, moving, and other parts that tend to wear excessively under therigors of operating conditions.

A further object of this invention is to develop a solid state voltageand current balanced system for substantially directly controlling theoperative positioning of an electrode.

Still a further object of this invention is to provide an efficient,effective, but flexible control system for individual electrodes, andparticularly one that enables the utilization of an alternatingelectrical energy source.

These and other objects of this invention will appear to those skilledin the art from the illustrated embodiment and the claims.

The invention makes use ofa circuit that has opposed circuit portionsthat are isolated from, but derive signals from, the direct energizationcircuit of an associated electrode and are normally balanced when theelectrode to be controlled is in a proper or desired working position,for example, in a position at which a suitable length of arc ismaintained between the tip of the electrode and the metal beingprocessed or melted or when the tip is in a properly submerged relationbeneath a slag blanket of a pool of molten metal. The system uses avoltage-sensitive circuit for lowering the electrode by means of areversible electric motor and a winch or winding drum to a position atwhich the top of the electrode, for example, makes contact with metal ina vessel and current starts to flow. The rush of current then actuates acurrent-sensitive circuit to reverse the motor and the winding drum toraise the electrode to a position at which a desired length of arc ismaintained or a desired resistance to current flow is provided.

The system is set in such a manner that the voltage and current circuitsbalance each other as long as the electrode is, for example, in a properare maintaining metal melting or a proper current flow heating position.

The signals derived from the current and voltage circuits are passedthrough full wave rectifiers to convert the alternating current intopulsating direct current. Such direct current flow is impressed, bymeans of a saturable core reactor, upon power supplied by independentalternating current sources employed for energization of the motor. Themotor energization current is in turn passed through fullwave rectifiersto convert the augmented alternating current thus supplied into directcurrent.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. 1 is a diagramatic vertical view in elevation showing a furnace insection provided with'three electrodes and representative means forraising one of them; and

FIGS. 2a and 2b comprise a schematic circuit diagram illustrating asystem or means coupled to a line representing one phase of athree-phase alternating current source that is being employed forenergizing an individual electrode; opposed voltage and current circuitsare shown employed with the energizing circuit of the individualelectrode in a coupled relation for maintaining the electrode in aproper operating position.

DESCRIPTION OF PREFERRED EMBODIMENT In FIG. 1, a conventionalthree-phase electric arc furnace is illustrated having three identicalelectrodes 16a, 17a, and 18a for meltingv a metal charge 21. Athree-phase alternating current source represented by busbar leads 10,11 and 12 is shown in FIG. 2a applied to the primary P ofa-delta-connected power transformer 15 through a circuit breaker 13. Thesecondary S of the transformer 15 has suitable flexible leads or cables16, 17 and 18 representing separate phases, each connected to one of thethree electrodes 16a, 17a and 18a. Since the control system for eachindividual electrode is the same, an exemplary or representative systemembodying the invention has been applied to the circuit for theelectrode 16a. The electrode 16a is shown in FIG. 1 suspended by a cable23 which interleaves over a pair of sheaves or pulley wheels 24a and 24band is wound and unwound on a winding drum or winch 22 that is driven bya shaft of a reversible electric motor M. The circuit system foroperating the motor M is illustrated particularly in FIG. 2a and FIG.2b.

When the'switch contacts of the circuit breaker 13 are closed,three-phase power is applied to the primary winding P of the transformer15. As a result, singlephase current is supplied from the secondary S ofthe transformer 15' to line 16, thus energizing electrode 16a.Assuming'that the electrode is in a raised, nonoperative position withrespect to metal ina vessel or furnace 20, a relatively high voltagewill be impressed between electrode 16a and the ground G from thesecondary winding S of the power transformer 15. As illustrated, avoltage isolating transformer 15 has its primary winding P connected bylead 16b to the line 16 through switch La for inductively applying thishigh voltage to voltage-sensing circuit A.

The primary winding P of the transformer 25 may have as shown a seriesof voltage selection taps for the line 16b, as controlled by individualselector switches 25a, 25b, and 250 and 25d so as to enable the controlof the voltage to be developed in circuit .A. As a safety feature, alimit switch 27 is placed in lead on line 26 that is connected from anopposite side of the transformer primary P to the ground G. The switch27 is actuated by the electrode 16a or by a suitable portion of theraising and lowering mechanism and serves to limit the maximum in ordown position to which the electrode 16a may be moved by the voltagecircuit A. The secondary S of the voltage transformer 25 is shownconnected by leads 28 and 29 through a variable control rheostat 35, toopposite sides of a full wave rectifier 30, preferably employingsemiconductor diodes. The positive terminal of the rectifier 30 isconnected to the primary winding of a saturable corereactor 33 bycircuit lead 31. This provides the controlled or manual setting of thelowering travel of electrode 16a.

The reactance winding P of saturable core reactor 33 is coupled throughan iron core to a secondary reactance winding 34 which is connected inseries with the primary P of transformer 36 having a stepdown ratio.

Single phase alternating current of, for example, 440

2 volts may be applied to leads 37 and 38 from an independent source ofelectricity to the series connected windings represented by thesecondary reactor winding 34 and the primary winding P of thetransformer 36. As a result, alternating current is imposed upon thePrimary P of transformer 36. As a result of primary winding P beingenergized, a resultant alternating potential is transferred through theiron core of transformer 36, into the secondary winding S of transformer36 on lines 40 and 41, which is then applied to the full wave rectifierbridge 42.

With full wave rectifier bridge 42 being energized, a potential ispassed and rectified, resulting in a current to the motor armature overline 44 through resistor 48 of a rectifier 46 then through lead 47, andthru the motor armature back to the bridge 42. Having energy, the motorarmature rotates, causing a downward movement of electrode 16a.

The electrode 16a having traveled downward, makes contact with scrap orcharge, 21, and causes a current to flow through electrode 16a and line16, which energizes current transformer in circuit B which in turn hasan alternating current output proportional to the current flowing inelectrode 16a. The output potential of transformer .60 is identified asCPI and CO (ground).

The output of the current transformer 60 is bridged by a fixed resistor61 in series with a variable resistor 62 to provide a variable load. Theprimary winding P of an isolating transformer 56 is connected across theresistors 61 and 62 and provides a 4 to 1 step down voltage at itssecondary winding S. Adjusting the loading across the primary winding Pof the transformer has the effect of setting the systemfor the desiredcurrent which is to be passed by the electrode.

The secondary winding S of the transformer 56 is shown connected toleads 63 and 64 through a variable control rheostat 69, and oppositesides of a full wave bridge rectifier 65. Negative and positiveterminals of the rectifier 65 are connected to opposite sides of aprimary winding of a saturable core reactor 54 by circuit leads 66 and67. The variable control rheostat 69, operates to control the potentialapplied to saturable core reactor 54.

The primary winding of the saturable core reactor 54 is coupled throughan iron core to a secondary winding 55 which is connected in series overline 55a with the primary winding P of the transformer 50. An Auxiliaryalternating current source is applied via leads 37 and 38 to transformer50. The same single-phase 440 volt source used to energize transformer36 may also be used to energize transformer 50. This series arrangementserves to vary the amplitude of the signal impressed on the primarywinding P of transformer 50. As in circuit A, momentary surges passedthrough the current control circuit B vary the reactance of winding 55and control the voltage delivered to the primary winding P of thetransformer 50 and the resulting voltag derived by its secondary windingS.

Motor energization circuit C includes secondary winding S of thetransformer 36 which is connected by leads 40 and 41 to opposite sidesof a full wave rectifier 42. The output terminals of the rectifier 42,as represented by leads 44 and 45, are respectively positive andnegative and are connected in conductive relation to opposite sides ofthe armature of reversible, direct current control motor M. Controlmotor M may have a permanent magnet field or a field winding F excited,

for example, by 230 volts D. C. A variable resistance 49 is shuntedacross the output terminals of the rectifier 42 for facilitating currentflow in an opposite direction in circuit C when current control circuitB has a predominating influence over the voltage circuit A.

Circuit C also includes secondary winding S of the transformer 50 whichis connected by leads 51 and S2 to the input terminals of a full waverectifier bridge 46. The output terminals of the rectifier 46, asrepresented by the lead 44 and 47, are respectively positive andnegative and are connected in conductive relation to opposite sides ofthe direct current motor M. A variable resistance 48 is shunted acrossthe output terminals of the rectifier 46 for facilitating current flowin an opposite direction in circuit C when voltage control circuit A hasa predominating influence over the current control circuit B.

The adjustment of the resistors 48 and 49 also regulates the amount ofdynamic braking applied in the motor armature circuit. This results fromthe fact that both resistors are connected in series across the armatureleads 45 and 47. Thus, when the voltage collapses or the system isshut-off, continued motion of the motor M is opposed by the load ofresistors 48 and 49, thereby providing the dynamic braking circuit.

As distinguished from the transformer 36 which is of a step down type,for example, converting 480/440 volts AC applied to its primary to 220volts AC at its secondary, with a corresponding proportional increase incurrent at its secondary, the transformer 50 is of a 1:1 ratio in thesense that 480/440 volts AC applied to its primary winding provides480/440 volts at its secondary winding. As a result, a potential at 440volts, for example, is applied to the full wave rectifier 46 as comparedto 220 volts as applied to the rectifier 42, which leaves a 220 voltdifferential. An additional input of about 220 volts is induced in motorenergization circuit C from the voltage circuit A via transformer 36when the electrode 16a is in a normal desired operating position. Thus,under normal operating conditions, the voltages derived from rectifiers42 and 46 are balanced i.e., of equal magnitude, but of oppositepolarity. The net current flow in circuit C is zero. However, when theelectrode has a greater than desired spacing with respect to the metalto be melted or heated, as when the electrode is at the top of itstravel or the charge melts away from the electrode, the voltage appliedby circuit A will be greater than 240/220 volts, such that an energizingflow of direct current will be applied to the armature of the motor M inthe direction indicated by the positive and negative sides at outputterminals or rectifier 42, thereby, effecting a lowering of theelectrode. On the other hand, when the current flow through the line 16to the electrode 16 a increases above a normal value represented by aselected desired position with respect to the metal 2], this will causethe direct current supplied from circuit B and by rectifier 46 to begreater than that supplied by the rectifier 42, such that the flow ofenergizing current to the armature of the motor M will be in an oppositedirection, as represented by the positive and negative sides applied tothe output terminals of the rectifier 46.

Thus, the motor M is driven in an opposite or reverse direction to thatdescribed previously and the electrode will be raised. It will be notedthat undesirable hunting" of the electrode caused by voltage or currentsurges in the supply lines is minimized by the feedback systems A and Binserted in circuit lines 670 and 67b, and lines 32 and 32a.

For controlling the bandwidth of response of the control system,feedback circuits Fa and F12 are utilized to provide bandwidth biasingof the lowering signals and raising signal, respectively. Each feedbackcircuit includes a rectifier bridge 72, 72a connected in circuit withthe primary windings 33 and 54 of voltage control circuit A and currentcontrol circuit B respectively. Thus, the inputs to rectifier 72 and 72aare proportional to the strength of the signals applied to the saturablecore reactors. Full wave rectifiers 72 and 72a may be of the bridgetype, and include resistances 73 and 73a shunted across the outputterminals to provide for a flow of current through the rectifier whichis opposite to that normally supplied. The outputs from the bias controlrheostats 59 and 59a, which may be the ribbon type, are fed to therectifiers 72 and 7211, respectively, over lines 58a and 58b, and lines57a and 57b. The control rheostats 59 and 59a are used to control thestrength of the signal coming from the secondary side of isolationtransformers 57 and 58. The input terminals of feedback circuit Fa areconnected by leads 63f and 64f to current control circuit B at leads 63and 64 respectively. Input terminals of the primary of isolationtransformer 58 are connected, via conductors 28f and 29f, to leads 28and 29 respectively of voltage control circuit A. The maximum output ofeach of the feedback circuits Fa and Pb can be about 12 volts DC. Theintroduction of the feedback signal into the voltage and current controlcircuits causes the signal strength there to be diminished, therebyreducing the corresponding signal in the motor energization circuit Cwhich reduces the imbalance in circuit C, which in turn causes motor Mto remain stable during short cycled variations of current.

To effect manual control of the electrode position independently of theautomatic control system, switches La and Lb are provided for loweringthe electrode and switches Ra and Rb are provided for raising theelectrode. Switch La is normally closed and switch Lb is normally open.When manual lowering of the electrode is desired, switch La is manuallyopened and switch Lb is manually closed. This isolates circuit A fromelectrode feed line 16 and causes the introduction of an independentsignal from an AC supply source, to be introduced'into circuit A throughswitch Lb. Likewise, switch Ra is normally closed and, when opened,isolates current transformer 60 and the associated circuitry fromcircuit B. The introduction of an independent signal source through theremainder of circuit B can then be accomplished by manually closingswitch Rb.

It will be apparent from the description of the illustrated embodimentthat there are no moving parts in the system of the invention from thestandpoint of, for example, relays, relay contracts, generators, etc. lnthis connection the system is a solid state system from the standpointof effecting an electrode position energization of the control motor M.The power supplied by the control circuits A or B augments powersupplied by transformers 36 and 50, such that minimum variations will besensitive in effecting the operation of the motor M, but need not inthemselves be the full energy sources for actuating the motor M.

Although an embodiment of the invention has been illustrated in thedrawings and described in this specification, it will be apparent tothose skilled in the art that variations, modifications, adaptions, andadditions may be made to the illustrated system or apparatus withoutdeparting from the spirit and scope of the invention.

What is claimed is: 1. A control system for adjusting the position ofanelectrode comprising means for sensing an electrical potential existingat the electrode and generating a signal in response thereto, means forsensing current flow through the electrode and generating a signal inresponse thereto; a reversible electric motor; means responsive torotation of the motor for moving the electrode in either of twodirections; a circuit for energizing the motor including a first meansfor causing current to flow in a first direction in the circuit and asecond means for causing electrical current to flow in the circuit in adirection opposed to the first direction; means for impressing a signalgenerated by the potentialsensing means on the first means; means forimpressing a signal generated by the current-sensing means on the secondmeans; said potential-sensing and generating means including anisolating transformer having a primary winding and a secondarywinding,'a rectifying means electrically connected to the secondarywinding of the isolating transformer, and a saturable core reactorhaving its primary winding electrically connected to the output of therectifying means; and saidcurrentsensing and signal generating meansincluding a current transformer, a rectifying means, an isolationtransformer electrically connecting the current transformer to therectifying means, and a saturable core reactor having a primary windingelectrically connected to output terminals of the rectifying means.

2. The electrode control system of claim 1 wherein the first meansincludes a first transformer and the secondary winding of the saturablecore reactorof the potential-sensing and generating is in series withthe primary winding of the first transformer, and wherein the secondmeans includes a second transformer and the secondary winding of thesaturable core reactor of the current sending and generating means is inseries with the primary winding of the second transformer.

3. The electrode control system of claim 2 wherein the first meansincludes rectifying means electrically connected to the secondarywinding of the first transformer and wherein the second means includesrectifying means connected to the secondary windings of the secondtransformer.

1. A control system for adjusting the position of an electrodecomprising means for sensing an electrical potential existing at theelectrode and generating a signal in response thereto; means for sensingcurrent flow through the electrode and generating a signal in responsethereto; a reversible electric motor; means responsive to rotation ofthe motor for moving the electrode in either of two directions; acircuit for energizing the motor including a first means for causingcurrent to flow in a first direction in the circuit and a second meansfor causing electrical current to flow in the circuit in a directionopposed to the first direction; means for impressing a signal generatedby the potential-sensing means on the first means; means for impressinga signal generated by the current-sensing means on the second means;said potential-sensing and generating means including an isolatingtransformer having a primary winding and a secondary winding, arectifying means electrically connected to the secondary winding of theisolating transformer, and a saturable core reactor having its primarywinding electrically connected to the output of the rectifying means;and said current-sensing and signal generating means including a currenttransformer, a rectifying means, an isolation transformer electricallyconnecting the current transformer to the rectifying means, and asaturable core reactor having a primary winding electrically connectedto output terminals of the rectifying means.
 2. The electrode controlsystem of claim 1 wherein the first means includes a first transformerand the secondary winding of the saturable core reactor of thepotential-sensing and generating is in series with the primary windingof the first transformer, and wherein the second meaNs includes a secondtransformer and the secondary winding of the saturable core reactor ofthe current sending and generating means is in series with the primarywinding of the second transformer.
 3. The electrode control system ofclaim 2 wherein the first means includes rectifying means electricallyconnected to the secondary winding of the first transformer and whereinthe second means includes rectifying means connected to the secondarywindings of the second transformer.